Contacts to connect devices together to form integrated circuits are generally formed by making openings in the surface of an insulating material disposed between the devices, and depositing a conductive metal, such as aluminum, into the openings and in contact with the devices. However, since aluminum melts at low temperatures, and reacts with the doped silicon which forms the devices, to cause spiking (migration) of the aluminum into the device, a barrier material is generally first deposited in the opening to prevent spiking. Thus a refractory metal and/or compound is used as a barrier layer. Suitable barrier materials include metal and compound layers of titanium, tungsten, tantalum, cobalt and the like. The efficacy of a titanium nitride layer as a barrier material is well known, and is representative of a good barrier material. However, since titanium and titanium nitride are not as conductive as aluminum, heating the substrate to form a more conductive silicide, such as titanium silicide (TiSi.sub.2), at the bottom of the opening, is also conventional.
Titanium silicide is generally formed by sputter depositing titanium in a physical vapor deposition (hereinafter PVD) chamber and then transferring the substrate to a rapid thermal anneal (RTA) chamber. The substrate is heated to elevated temperatures, such as between about 500-900.degree. C., to react the titanium with the silicon substrate to form a conductive titanium silicide layer at the substrate. However, this method requires more than one processing chamber and the substrate may be exposed to oxygen and particulates during transfers from one chamber to another. Further, the step coverage in a conventional DC magnetron PVD chamber for small diameter (0.4 micron for example) high aspect ratio (AR&gt;3:1) openings is less than 10%.
Recently, improvements have been made to conventional sputtering chambers that permit the formation of a high density plasma in the chamber. Particles that are sputtered from a target pass through a high density plasma region where they are ionized to form positively charged ions. The substrate, which rests on a biased substrate support, is negatively charged. This causes a more vertical deposition onto the substrate when ions impact the substrate, and improved filling of small diameter, high aspect ratio openings. Step coverage can be increased by about four times in such a chamber.
FIG. 1 is a schematic cross sectional view of such a chamber, known as an ionized metal plasma, or IMP chamber. Referring to FIG. 1, the IMP chamber 170 includes a conventional target 172 mounted on a top wall 173 of the chamber 170. A rotating magnet shown as 176, 178 is mounted over the top of the chamber 173. A substrate support 174, bearing a substrate 175 thereon, is mounted parallel to and spaced from the target 172. A source of DC power 180 is connected to the target 172 and a source of RF power 182 is connected to the substrate support 174. A controller 200 regulates gas flows. A coil 186 is mounted inside the chamber 170 between the target 172 and the substrate support 174, and is connected to a source of RF power 188. Gases in vessels 192, 194 are metered to the chamber by means of flow valves 196, 198.
The pressure in the chamber is maintained by a cryogenic pump 190 through an inlet 191 via a three position gate valve 199. Providing that the pressure in the chamber is fairly high, i.e., about 30-40 millitorr, the internal inductively coupled coil 186 provides a high density plasma in the region between the sputtering cathode or target 172 and the substrate support 174. Thus sputtered target atoms become ionized and positively charged as they pass through the high density plasma region. They are attracted by the negatively biased substrate and thus impact the substrate in a more vertical direction than occurs in conventional PVD chambers. The IMP chamber is generally operated at higher pressures than conventional sputtering chamber, i.e., 30-40 millitorr rather than 1-5 millitorr for conventional sputtering chambers.
Using such a chamber, an in situ deposition of titanium while simultaneously heating the substrate to about 650.degree. C. was tried in order to form titanium silicide in the IMP chamber. An in situ deposition would be advantageous because it eliminates having to transfer the substrate to a separate RTA chamber or separate system, thus reducing the processing time and thus the costs of producing a contact. Further, there is less danger from contamination of the substrate due to additional transfer and handling of the substrate.
A study was performed in the IMP chamber using a blank silicon wafer. The deposition heater temperature for the wafer was set at 550-650.degree. C., DC power was 1 to about 5 kW, RF power to the coil was set at 1-3 kW and the pedestal bias was set at 5-150 volts. The formation of titanium silicide was confirmed.
However, when the study was repeated using a patterned wafer, this attempt was unsuccessful because a void formed in the silicon beneath the silicide. The result is shown in FIG. 2. FIG. 2 is a TEM photograph of an opening in a silicon oxide layer on a silicon substrate partially filled with titanium in an IMP chamber and heated to 650.degree. C. A void in the silicon is clearly seen at 20.
Considering the possibility that contamination on the surface of the silicon substrate caused or contributed to formation of the void, either with native oxide, carbon residues from wet etching, and/or residues from polysilicon or silicon oxide etching, the bottom of the opening was cleaned by both dry and wet etching; however, little improvement was noted, and voids were still formed in the silicon.
However, a method of depositing titanium on a silicon wafer and forming a silicide therefrom in the same chamber has such potential for lowering costs and improving quality, that work has continued to find a method for forming titanium silicide in situ in an IMP chamber without forming voids in the silicon.